The present invention relates generally to an electrorheological (ER) fluid force transmission and conversion device and more particularly provides an improved ER fluid clutch or brake in which the ER fluid is isolated within a working chamber and torque transmission to the output is accomplished by magnetic coupling to eliminate the need for a dynamic fluid seal. Self-contained power generation may also be accomplished to provide the required electric field for modulation of fluid yield strength caused by the ER effect.
Significant progress has been made in the development of low cost, reliable electrorheological fluids. In general, ER fluids consist of suspensions of very fine particles in a dielectric liquid media. Such fluids were first referred to as "electroviscous" because of their apparent viscosity changes in the presence of an electric field. A better understanding of these types of compositions has revealed that the phenomenon being observed is a change in the minimum stress required to induce shear of the fluid, while the actual viscosity remains generally constant. Accordingly, these effects are better described in terms of the total rheology of the composition, and as such are now more commonly referred to as "electrorheological" fluids. In the absence of an electric field, ER fluids exhibit Newtonian flow characteristics; their shear rate is directly proportional to shear stress. However, when an electric field on the order of 10.sup.3 volts per millimeter is applied, a yield stress phenomenon occurs such that no shearing takes place until the shear stress exceeds a yield value which rises with increasing electric field strength. This result can appear as an increase in apparent viscosity of several orders of magnitude.
Commercially realizable systems employing these fluids include variable clutch or brake assemblies. Early torque transmission systems departing from traditional mechanical clutch and brake assemblies utilized magnetic particles suspended in a liquid to offer the possibility of progressive control of the torque in response to the magnetic field applied. The application of a magnetic field cause the magnetic particles to adhere to one another between the mechanical drive elements and therefore varies the frictional force between them. However, the power required to generate an appropriate magnetic field and the physical size of the components needed for these devices present significant limitations on the ability to transmit torque.
Electric field responsive torque transmitting devices which take advantage of the ER effect possess the considerable advantage of very rapid response to changes in the applied field, previously unobtainable with magnetic fluid devices. Electric field responsive devices also may be constructed without heavy and expensive electromagnetic coils. As in U.S. Pat. No. 2,417,850, 2,886,151 and 4,664,236, disclose illustrative ER fluid clutch and brake mechanisms that offer the possibility of progressive and continuous control of torque in response to variation in the electric field across the fluid. Electrorheological fluids as applied to the development of mechanical system of this nature offer the potential for providing rapid and reversible response characteristics, with typical response times being on the order of one millisecond.
While ER fluid torque transmission devices provide for variable and responsive control superior to conventional viscous shear and other clutches as well as magnetic fluid arrangements, existing ER fluid torque transmission devices have heretofore been less than ideal. The torque limits of ER fluid devices are constrained by voltage potential and interactive surface area for ER fluid shear. Efforts to maximize the interactive surface area while maintaining the overall volume of the device to a minimum have not been entirely satisfactory, especially in view of cost prohibitive and inefficient manufacturing and assembly alternatives.
Typically, clutches require reliable performance under adverse and often unpredictable conditions of vibration, impact and other adverse conditions. Isolation or containment of the ER fluid within the system is problematic under such conditions especially in view of the dynamics of assembly components. The problem of maintaining system integrity without fluid leakage is exacerbated by temperature and pressure extremes experienced under normal working conditions. Other inherent shortcomings of existing systems include the need for responsive control. In order to take full advantage of instantaneous variability in ER fluid shear strength, command of electric field potential must be contemporaneously responsive to the system state and changing torque requirements. Further, an external power source or battery for supply of electrical energy to the system has in the past been required. An electric potential on the order of 10.sup.3 volts is typically required to create the electric field for implementation of the ER effect. The power requirements mentioned may limit the effectiveness of such devices for self-contained application.
It is accordingly an object of the present invention to provide an ER fluid torque transmission and conversion device which eliminates or substantially minimizes the above mentioned and other problems and limitations typically associated with ER fluid clutch and brake assemblies.